Most spectroscopy systems fall into one of two categories. They can be tunable source systems that generate a wavelength tunable optical signal that is scanned over a wavelength scan band. A detector is then used to detect the tunable optical signal after interaction with the sample. The time response of the detector corresponds to the spectral response of the sample. Such systems are typically referred to as pre-dispersive. Alternatively, a tunable detector system can be used. In this case, a broadband optical signal is used to illuminate the sample. Then, signal from the sample is passed through an optical bandpass filter or grating, which is tuned over the scan band such that a detector time response detector array response is used to resolve the sample's spectrum. Such systems are typically referred to as post-dispersive.
Between tunable source and tunable detector systems, tunable source systems have some advantages. They can have a better response for the same optical power transmitted to the sample. That is, tunable detector systems must illuminate the sample with a broadband signal that covers the entire scan band. Sometimes, this can result in excessive sample heating and power consumption at the source, making the systems inefficient. In contrast, at any given instant, tunable source systems only generate and illuminate the sample with a very narrow band within the scan band.
Further, tunable source systems have advantages associated with detection efficiency. Relatively large detectors can be used to capture a larger fraction of the light that may have been scattered by or transmitted through the sample, since there is no need to capture light and then collimate the light for transmission through a tunable filter or to a grating, for example.
A number of general configurations are used for tunable source spectroscopy systems. The lasers have advantages in that very intense tunable optical signals can be generated. A different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample.
Historically, most tunable lasers were based on solid state or liquid dye gain media. While often powerful, these systems also have high power consumptions. Tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources. One configuration uses semiconductor optical amplifiers (SOAs) and microelectromechanical system (MEMS) Fabry-Perot tunable filters, as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety. In other examples, intra cavity gratings are used to tune the laser emission.
In commercial examples of the broadband source/tunable filter source configuration, the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example. More recently, some of the present inventors have proposed a tunable source that combines edge-emitting, superluminescent light emitting diodes (SLEDs) and MEMS Fabry-Perot tunable filters to generate the tunable optical signal. See U.S. patent application Ser. No. 10/688,690, entitled Integrated Spectroscopy System, filed on Oct. 17, 2003, by Atia, et al., which is incorporated herein by this reference in its entirety. The MEMS device is highly stable, can handle high optical powers, and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example.
Moving from standard diode arrays and tungsten-halogen bulbs to edge-emitting devices such as superluminescent light emitting diodes (SLED), other edge emitting diodes including lasers, and semiconductor optical amplifiers (SOA) has the advantage that higher optical powers can be achieved.
One characteristic of these edge-emitting semiconductor devices such as SLEDs, diode lasers, and SOAs is that they usually emit single spatial mode light that is highly polarized. This is due to the nature of the semiconductor gain medium. The waveguide ridges are narrow preventing gain in higher order spatial modes. Also, current is usually injected from a top electrode through a quantum well structure to the bottom electrode. Thus, the gain medium is not circularly symmetric around the optical axis and thus light from these devices is usually highly polarized. Most often, it emits light in only a single polarization.
Even vertical surface emitting laser (VCEL) devices, where the gain region is more symmetric, tend to be highly polarized. This is because invariably one of the polarization modes encounters more loss so that the device runs in the other mode. In fact, it is common to fabricate the devices so that there is a strong preference for one of the modes to remove uncertainly as to in which mode the device operates.
Thus, in these systems, the broadband signal or the tunable signal that is transmitted to the sample is usually highly polarized and usually has only a single spatial mode.
For applications requiring a high signal-to-noise operation, when the source is highly polarized, polarization dependent loss is often a significant problem. The optical link between the tunable signal or broadband signal source and the sample and between the sample and the detector will have PDL. Moreover, this PDL may be dynamic over time especially in response to mechanical vibration or other changes to the fiber links or other optical elements in the path between the source and sample and from the sample to the detector. This PDL, in view of the highly polarized nature of the light from these semiconductor sources, can introduce spectral distortion in the measured signal and can detrimentally impact the signal-to-noise ratio and thus spectral performance of these systems.
To address the PDL problems U.S. patent application Ser. No. 11/018,687, filed Dec. 21, 2004, Polarization Controlling Fiber Probe for Semiconductor Source Spectroscopy System, by Flanders, et al., which is incorporated herein in its entirety by this reference, describes a semiconductor source spectroscopy system. It relies on using polarization control between the source and the sample and/or the sample and the detector.